Photoemission Observations ofπ−dBonding and Surface Reactions of Adsorbed Hydrocarbons on Ni(111)

Ionization energies for chemisorbed acetylene, ethylene, and benzene exhibit $\pi$-orbital bonding shifts (∼ 0.9-1.5 eV) as well as large nonchemical-bonding relaxation shifts of ∼ 1-3 eV for both $\pi$ and $\sigma$ orbitals. Dehydrogenation of chemisorbed ethylene to acetylene for $T\gtrsim 230$°K is directly observed. We estimate $\pi -d$ bonding interaction strengths and chemisorption energies from spectroscopic energy levels and predict that this surface reaction becomes exothermic because of $\pi -d$ bonding.